1. Field of the Invention
The present invention relates to nuclear fission reactors and more particularly pressurized water reactor (PWR) fuel assemblies. The present invention also relates to a method of forming a PWR fuel assembly to obtain enhanced fuel utilization.
2. Description of the Prior Art
The generation of a large amount of heat energy by nuclear fission in a nuclear reactor is well known. This energy is dissipated as heat in elongated nuclear fuel rods. A plurality of nuclear fuel rods are typically grouped together in an array to form separately removable fuel assemblies. A number of such nuclear fuel assemblies are generally arranged in a matrix to form a nuclear core capable of a self-sustained, nuclear fission reaction. The core is typically submersed in a fluid, such as light water, that serves as a coolant for removing heat from the nuclear reactor fuel rods and as a neutron moderator.
In a typical fuel assembly, the fuel rods contain a fuel zone which typically comprises a stack of enriched nuclear fuel pellets encased in a length of tubular cladding material. A fission gas plenum zone is typically disposed in the cladding, above the fuel zone for collecting fission product gases generated within the fuel rods during burnup.
In some prior designs, such as that described in U.S. Pat. No. 3,425,908 the fuel rods are held in a spaced, lateral array but are axially displaced from each other to form a tranverse cross section in the shape of a paraboloid or spherical dome. In this configuration, the fission gas plenum zones are axially displaced with respect to each other but are all located at the top of the fuel rods as they are held in the fuel assembly.
U.S. Pat. No. 3,586,603 describes a fuel assembly having laterally offset fission gas plenum zones which permit the reduction in the core coolant pumping requirements. In this configuration, all of the fission gas plenum zones remain adjacent to each other at the top of fuel assembly.
In U.S. Pat. Nos. 4,125,433 and 4,130,460 fuel assemblies for nuclear fuel reactors are described having centrally disposed fuel rods with fission gas plenums extending in one axially direction from the fuel zone while the fission gas plenum zones of the pheripheral rods are grouped together and extend in the opposite axial direction. The fuel zones of all the fuel rods of the fuel assemblies of both references are "coextensive, having no offset.
In U.S. Pat. No. 3,679,545 there is described a nuclear fuel rod or control rod design which utilizes resilient, corrugated spacer members disposed within a cladding tube to establish plenum zones. The plenum zones may be located at the top, at the bottom, within a middle region, or any combination of the above within the fuel or control rod.
In U.K. Pat. No. 923,633 there is disclosed a method of charging a heterogeneous nuclear reactor to obtain better fuel utilization by dividing the fuel lattice to axially zones and transporting fuel rods from zone to zone to equalize the fuel burnup.
In co-pending commonly assigned application Ser. No. 609,252 filed concurrently herewith (Freeman, Wilson, and Knott) a boiling water reactor (BWR) fuel assembly is disclosed which operates to lower fuel cycle costs associated with BWR operation by providing a fuel assembly which is top to bottom reversible and which has some fuel rods with fission gas plenum zones at the top of the fuel assembly and some at the bottom to thereby increase the effective active length of the core and to increase the H/U ratio at the end of the core.
None of this prior art provides a method or apparatus for enhancing fuel utilization and for reducing neutron leakage in a PWR fuel assembly.